1. Field of the Invention
The present invention relates to an information recording medium on or from which information is recorded or reproduced by an optical or an electrical system and a method for manufacturing the same as well as a target to be used for the manufacturing method.
2. Related Background Art
An example of optical information recording media is a phase-change information recording medium in which information is recorded, rewritten, or erased optically using a laser beam. The recording, erasure, and rewriting in the phase-change information recording medium are performed through reversible changes of a phase change material between crystalline and amorphous states in a recording layer thereof. Generally, information is recorded by irradiating an information recording medium with a high power laser beam to heat the recording layer to a higher temperature than its melting point so as to melt the irradiated region, and then cooling it rapidly to form an amorphous phase. On the other hand, information is erased by irradiating the information recording medium with a lower power laser beam than that used for recording to heat the recording layer to a temperature that is higher than its crystallization temperature but is lower than its melting point to increase the temperature of the recording layer and then cooling it slowly to form a crystalline phase and thereby to erase information. These crystallized region and amorphous region thus formed differe in reflectance from each other and thereby information can be reproduced. Accordingly, in order to obtain high quality reproduced signals, it is necessary to increase the difference in reflectance.
Examples of the phase-change information recording medium include currently commercialized Blu-ray Disc media. The Blu-ray Disc is a medium that can respond to digital high-definition broadcasting. The Blu-ray Disc has a storage capacity of 25 GB (one layer) or 50 GB (two layers on one side) and a transfer rate of 36 Mbps (1× speed). Examples of the recording layer materials contained in the 1× speed rewritable Blu-ray Disc media include one having a composition located on a line extending between Ge50Te50 and Sb40Te60 (see JP63 (1988)-225934 A) and one having a composition located on a line extending between Ge50Te50 and Bi40Te60 (see JP 63 (1988)-225935 A). The composition located on the line extending between Ge50Te50 and Bi40Te60 is obtained by substituting Sb with Bi in the composition located on the line extending between Ge50Te50 and Sb40Te60.
Further increases in capacity and transfer rate are desired for the future and various technologies according thereto are being studied. Providing an information recording medium with two or more information layers on one side is considered as one of the technologies for increasing the capacity thereof. In the case of using this technology, the reflectance change in each recording layer is reproduced with a laser beam that enters from one side of an information recording medium 1. Therefore the information layers that are located nearer to the laser beam incident side are required to have a higher transmittance that allows them to transmit the laser beam. Accordingly, in the information layer located on the laser beam incident side, the recording layer has to be thin to have higher transmittance. However, when the thickness of the recording layer is reduced, it tends to be difficult for atoms to migrate. This results in a reduction in crystallization rate of the recording layer. Therefore when the recording layer is made thin, a phase change material with an increased crystallization rate accordingly has to be employed.
Furthermore, a resultant higher transfer rate reduces the period of time for which the recording layer is irradiated with a laser beam. Accordingly, the time required for the recording layer to be crystallized has to be shortened. Therefore a phase change material that has a further increased crystallization rate is required.
As described above, in order to increase the speed of information recording media, the recording layers thereof have to have increased crystallization rates. Accordingly, it is necessary to increase the composition ratio of Sb40Te60 in the composition on the line extending between Ge50Te50 and Sb40Te60 and the composition ratio of Bi40Te60 in the composition on the line extending between Ge50Te50 and Bi40Te60. According to the experiments made by the present inventors, it was confirmed that in the case of using compositions on those lines, the rate at which recorded marks were erased was low even when a composition was used that had a crystallization rate increased corresponding to high linear velocity recording.
Conceivably, a cause of a low erasure rate is distortion (absorption distortion) of marks that results from the difference in optical absorptance between a crystalline region and an amorphous region. In a current High-to-Low structure (a structure in which when recording is performed in an unrecorded region, the reflectance of the recorded region is lower than that of the unrecorded region), the absorptance of a crystalline region is lower than that of an amorphous region. Therefore, in the same track of an information recording medium, a crystalline region and an amorphous region are different in meltability, and thus in conditions for forming a mark, from each other. These differences result in a decrease in erasure rate. The absorption distortion as described above appears prominently in high-speed recording in which the laser beam irradiation time is shorter. It also has been proved by the present inventors that in an information recording medium on which recording is performed using a blue-violet laser, it is difficult to design it to have a small difference in absorptance between a crystalline region and an amorphous region so as to reduce the effect of the absorption distortion. Therefore, a recording layer material having another crystallization mechanism is required for high-speed recording.
Furthermore, in the case of using the composition whose crystallization rate has been increased on the line extending between Ge50Te50 and Sb40Te60 or the line extending between Ge50Te50 and Bi40Te60, optical variations in refractive index and extinction coefficient between the crystalline region and the amorphous region are reduced, and thereby the difference in reflectance between the crystalline region and the amorphous region of the information recording medium is reduced. Accordingly, the quality of reproduced signals is deteriorated. Furthermore, in the composition whose crystallization rate has been improved as described above, the crystallization temperature also decreases. That is, this leads to deterioration in signal storage stability. These points also were proved through the experiments made by the present inventors.
On the other hand, when the number of information layers to be increased, which accompanies an increase in capacity, recording layer materials are required that are employed according to the crystallization rate needed in each layer as described above. Therefore when a recording layer is formed from one target, for example, in an information recording medium having four information layers, a maximum four types of targets are required. Accordingly, the cost for targets is high and additional cost is required. The additional cost is cost that accompanies an increase in the number of film formation chambers. Therefore, it is difficult to reduce the cost of the information recording medium.